US8062751B2 - Low biofouling filtration membranes and their forming method - Google Patents
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- B01D67/0002—Organic membrane manufacture
- B01D67/0023—Organic membrane manufacture by inducing porosity into non porous precursor membranes
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- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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- B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
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- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
- B01D71/06—Organic material
- B01D71/30—Polyalkenyl halides
- B01D71/32—Polyalkenyl halides containing fluorine atoms
- B01D71/34—Polyvinylidene fluoride
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- B01D—SEPARATION
- B01D2323/00—Details relating to membrane preparation
- B01D2323/38—Graft polymerization
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2325/00—Details relating to properties of membranes
- B01D2325/18—Membrane materials having mixed charged functional groups
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2325/00—Details relating to properties of membranes
- B01D2325/28—Degradation or stability over time
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
- B01D61/14—Ultrafiltration; Microfiltration
- B01D61/145—Ultrafiltration
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/31504—Composite [nonstructural laminate]
- Y10T428/31536—Including interfacial reaction product of adjacent layers
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/31504—Composite [nonstructural laminate]
- Y10T428/3154—Of fluorinated addition polymer from unsaturated monomers
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/31504—Composite [nonstructural laminate]
- Y10T428/3154—Of fluorinated addition polymer from unsaturated monomers
- Y10T428/31544—Addition polymer is perhalogenated
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/31504—Composite [nonstructural laminate]
- Y10T428/31855—Of addition polymer from unsaturated monomers
- Y10T428/31935—Ester, halide or nitrile of addition polymer
Definitions
- the present invention is generally related to low biofouling filtration membranes, and more particularly to graft branched polymers or copolymers containing zwitterionic groups on fluorine-based filtration membranes or films for the purpose of anti-biofouling.
- Proteinaceous biomolecules are highly complex, containing both hydrophilic and hydrophobic regions. These biomolecules are highly conformable and adaptable toward adsorption to surfaces having hydrophobic moieties thereat. Therefore, many hydrophilic surfaces are used to reduce protein adsorption. However, these surfaces are often not sufficient to prevent the undesirable adhesion of cells, bacteria, or other microorganisms. Even a small amount of proteins on a surface can lead to the adhesion and propagation of unwanted fouling. For example, fibrinogen adsorption less than 5-10 ng/cm 2 is needed to inhibit platelet adhesion for blood compatibility, and superlow fouling surfaces are required for these applications.
- Nonfouling materials or “superlow fouling materials”.
- Poly(ethylene glycol) (PEG) or oligo(ethylene glycol) (OEG) modified surfaces have been extensively studied to resist protein adsorption. The steric exclusion effect was considered as one of the reasons for PEG polymers to resist protein adsorption.
- PEG or OEG group decomposes in the presence of oxygen and transition metal ions found in most biochemically relevant solutions.
- PC-based polymers or surfaces have been shown to decrease protein adsorption. They are considered as biomimetic fouling-resistant materials since they contain phosphorylcholine headgroups, which are found in the outside layer of cell membranes. The hydration of PC-based materials is also thought to be the reason for their resistance to protein adsorption. However, it is desirable to develop new materials other than PC for applications requiring long-term material stability due to the tendency of the phosphoester group to be hydrolyzed. In addition, PC monomers, such as 2-methacryloyloxyethyl phosphorylcholine (MPC), are moisture sensitive and not easy to synthesize and handle.
- MPC 2-methacryloyloxyethyl phosphorylcholine
- new method for forming low biofouling filtration membranes is provided that substantially overcomes the drawbacks of the above problems mentioned from the conventional system.
- One object of the present invention is to use sulfobetaine polymers for surface modification of fluorine-based porous membranes. Similar to phosphorylcholine-based polymers, sulfobetaine polymers belong to polybetaine polymers, in which both cationic and anionic groups are on the same monomer residue. Compared with MPC, sulfobetaine methacrylate (SBMA) is easier to synthesize and handle.
- SBMA sulfobetaine methacrylate
- Another object of the present invention is to control the highly polar sulfobetaine monomers grafting from the chemical inert, hydrophobic fluro-based polymers.
- the advantages of the present invention are obtained via, sequential ozone surface activation and surface-initiated polymerization of an appropriate functional monomer with halide groups, and sulfobetaine polymers are then grafted from the halide groups on surface through atom transfer radical polymerization (ATRP).
- ATRP atom transfer radical polymerization
- Still another object of the present invention is to apply the fluorine-based filtration membranes with grafted zwitterionic group in protein separation.
- the filtration experiments for bovine serum albumin (BSA) separation and two types of plasma protein (albumin and globulin) separation revealed that irreversible membrane fouling was remarkably reduced due to the incorporation of zwitterionic sulfobetaine group from SBMA polymer.
- the cyclic filtration tests for albumin yield an extremely low irreversible membrane fouling ratio (R ir ) of 13% in the first cycle and apparently no irreversible fouling is found in the second cycle.
- a more stringent test is carried by passing the ⁇ -globulin solution.
- the virgin PVDF membrane is continuously fouled by ⁇ -globulin after 3 cyclic operations, but the polySBMA modified membrane has a R ir value as low as 4.7% in the third cycle. Therefore, this present invention does have the economic advantages for industrial applications.
- the present invention discloses a method for forming a low biofouling filtration membrane.
- an ozone treatment is performed to a fluorine-based porous membrane to introduce peroxides on surface.
- a first grafting polymerization is initiated from the peroxides, and functional monomers are polymerized to introduce halide groups on surface.
- a second grafting polymerization is initiated from the halide groups, and macro-monomers are polymerized to introduce zwitterionic group on surface, so as to form the low biofouling filtration membrane.
- FIG. 1 is schematic illustration of the preparation process of the PVDF-g-PBIEA-g-PSBMA UF membranes via surface copolymerization—(a) a PVDF UF membrane cleaned via sonic in double distilled water at 25° C., (b) the PVDF UF membrane pretreated with a continuous stream of O 3 /O 2 mixture in IPA at 25° C., (c) the ozone preactivated PVDF UF membrane incubated in a IPA solution containing PBIEA macroinitiator monomer at 80° C., (d) the PVDF-g-PBIEA membrane incubated in a methanol solution containing PSBMA macromonomer at 40° C.;
- FIG. 2 is XPS C 1S core-level spectra of (a) the virgin PVDF UF membrane, and (b) the PVDF-g-PBIEA UF membrane; High resolution XPS spectra of Br 3d region of (c) the PVDF-g-PBIEA UF membrane;
- FIG. 3 is FT-IR spectra of (a) the virgin PVDF, (b) the PVDF-g-PBIEA, and the PVDF-g-polySBMA membranes with polySBMA grafting density of (c) 0.18 mg/cm 2 and (d) 0.4 mg/cm 2 .
- FIG. 4 shows effect of SBMA content in the reaction solution on the surface grafting density and water contact angle of the prepared PVDF UF membranes.
- FIG. 5 are SEM photographs of surface morphology of the prepared PVDF UF membranes with polySBMA grafting amount of (a) 0.0 mg/cm 2 prepared by the wet phase-inversion process, (b) 0.0 mg/cm 2 with ozone pretreatment for 30 min at 25° C., (c) 0.18 mg/cm 2 , and (d) 0.4 mg/cm 2 . All images with magnification of 10000 ⁇ .
- FIG. 6 shows BSA and ⁇ -globulin adsorption amount on the surface of the prepared PVDF UF membranes as a function of polySBMA grafting density. All membranes were incubated in 5 mL of 1.0 mg/mL protein in PBS solution for 24 h at 37° C.
- FIG. 7 shows time-dependent flux of the PVDF UF membranes grafted with different amounts of polySBMA.
- Ultrafiltration process was operated at a pressure of 1.0 atm, a temperature of 25° C. and a stirring speed of 300 rpm.
- the BSA concentration is 1.0 mg/mL in PBS solution.
- FIG. 8 shows effect of polySBMA grafting density on flux recovery ratio (FR w,1 ) and fouling ratio (R: R t,1 , R ir,1 and R r,1 ) in the first cycle of the filtration test using BSA as the tested protein.
- FIG. 9 shows time-dependent of (a) recycling flux and (b) rejection ratio for the virgin PVDF UF membrane and PVDF-g-polySBMA membrane grafted with 4.0 mg/cm 2 PSBMA polymers, respectively. All process was operated with three cycles of BSA solution ultrafiltration in the room temperature.
- FIG. 10 shows Time-dependent of (a) recycling flux and (b) rejection ratio for the virgin PVDF UF membrane and PVDF-g-polySBMA membrane grafted with 0.4 mg/cm 2 polySBMA, respectively. All process was operated with three cycles of ⁇ -globulin solution ultrafiltration in the room temperature.
- FIG. 11 shows the comparison of water flux recovery during the i th cycle between the virgin PVDF UF membrane and PVDF-g-polySBMA membrane grafted with 0.4 mg/cm 2 polySBMA for (a) BSA solution and (b) ⁇ -globulin solution.
- PVDF Poly(vinylidene fluroride)
- MF microfiltration
- UF ultrafiltration
- NF nanofiltration
- One of the most important requirements for PVDF membranes in biomedical applications is to reduce the nonspecific adsorption of biomolecules when living systems encounter hydrophobic membrane surfaces.
- bio-fouling of membranes prepared from hydrophobic materials will lead to a change in biomolecular selectively decreasing the permeate flux with time, especially in the filtration of protein, platelet or cell-containing solutions.
- an ideal anti-fouling membrane should possess the excellent mechanical bulk properties of a hydrophobic material, such as PVDF, and the anti-fouling characteristics of a hydrophilic surface on the membrane surface and pores.
- a low biofouling polymeric composite film is disclosed, the composite film is applicable in the following applications: separation of peptides and protein; purification of human blood without Virus or Leukoocytes; separation of microbial from waste water; preservation, purification, concentration or separation of stem cells.
- the polymeric composite film comprises an activated fluorine-based substrate (preferred activated by ozone treatment), and a layer of branched polymer containing zwitterionic functional groups formed on the fluorine-based substrate via surface grafting.
- surface grafting is a two-step polymerization.
- halide containing monomers such as: 2-(2-bromoisobutyryloxy) ethyl acrylate or 2-(2-choloisobutyryloxy) ethyl acrylate.
- the second step is Atom Transfer Radical Polymerization through the halide containing monomers, and in the second step, monomers containing zwitterionic functional groups are used to form a grafting polymer.
- the content of zwitterionic functional groups in the grafting polymer is equal to or more than 10 wt %.
- the grafting density and chain length of polymer formed from the macro-monomers is at least 0.3 chains/nm 2 and 50 units, preferably above 0.5 chains/nm 2 and 100 units.
- the zwitterionic functional groups comprises one of the group consisting of: phosphobetaine, sufobetaine, carboxylbetaine, and their “derivatives”.
- the tern “derivatives” means phosphobetaine, sufobetaine, or carboxylbetaine can be in the form of mixed charged compounds from equal molar ratio of positive and negative groups.
- Positively charged compounds can be Aminoethyl methacrylate hydrochlorides, 2-(Dimethylamino)ethyl methacrylate or 2-(Methacryloyloxy) ethyl trimethylammonium chloride.
- Negatively charged compounds can be 2-Carboxyethyl acrylate or 3-Sulfopropyl methacrylate.
- a method for forming a low biofouling polymeric composite film is disclosed.
- an ozone treatment is performed to a fluorine-based membrane to form a first intermediate, on whose surface peroxides (alkyl-peroxide and hydroxyl-peroxide) are formed as a result of reaction of the fluorine-based membrane surface with ozone.
- a first grafting polymerization is initiated from the peroxides of the first intermediate, wherein each functional monomer comprises at least one acrylic group and at lest one halide group (chloride or bromide is preferred), the acrylic group of the functional monomer reacts with the peroxide of the first intermediate, and the functional monomers polymerize with each other by their acrylic group, so as to form a second intermediate with halide groups on surface.
- the preferred functional monomer is 2-(2-bromoisobutyryloxy) ethyl acrylate (BIEA).
- BIEA 2-(2-bromoisobutyryloxy) ethyl acrylate
- the temperature of the first grafting polymerization is higher than the peroxide decomposition temperature (higher than 70° C.).
- a second grafting polymerization is initiated from the halide groups of the second intermediate, wherein each macro-monomer comprises at least one acrylic group and at lest one zwitterionic group, the acrylic group of the macro-monomer reacts with the halide group of the second intermediate, and the macro-monomers polymerize with each other by their acrylic group, so as to form the low biofouling polymeric composite film with grafted zwitterionic group.
- the preferred macro-monomer is sulfobetaine acrylate or sulfobetaine alkylacrylate.
- the grafting density and chain length of polymer formed from the macro-monomers is at least 0.3 chains/nm 2 and 50 units, preferably above 0.5 chains/nm 2 and 100 units.
- the preferred fluorine-based membrane is porous membrane, and can be classified into one of the three categories: micro-filtration membrane, ultra-filtration membrane, and nano-filtration membrane
- the material of the fluorine-based membrane comprises one of the group consisting of: polyvinylidene fluoride (PVDF), copolymers of tetrafluoroethylene and perfluoro(propyl vinyl ether), copolymers of tetrafluoroethylene and perfluoro-2,3-dimethyl-1,3-dioxole, copolymers of tetrafluoroethylene and vinyl fluoride, poly(vinyl fluoride), poly(vinylidene fluoride), polychlorotrifluorethylene, vinyl fluoride/vinylidene fluoride copolymers, and vinylidene fluoride/hexafluoroethylene copolymers.
- PVDF polyvinylidene fluoride
- VDF polyvinylidene fluoride
- the surface of fluorine based polymer is activated by ozone treatment.
- the ozone concentration ranges from 5 to 50 g/m 3
- the duration of ozone treatment ranges from 5 to 60 minutes.
- both the first grafting polymerization and the second grafting polymerization are controlled/living free radical polymerization, and Atom Transfer Radical Polymerization (ATRP) is preferred.
- [2-(Methacryloyloxy) ethyl]dimethyl(3-sulfopropyl)-ammonium hydroxide (sulfobetaine methacrylate, SBMA) macromonomer was purchased from Monomer-Polymer & Dajac Laboratories, Inc. Copper(I) bromide (99.999%), 2-bromoisobutyryl bromide (BIBB, 98%), pyridine (98%), 2-Hydroxyethyl acrylate (97%), 2,2′′-bipyridine(BPY, 99%), and triethylamine(99%) were purchased from Sigma-Aldrich.
- Isopropyl alcohol (IPA, 99%) was obtained from Sigma-Aldrich and was used as a solvent for the ozone treatment and graft copolymerization.
- NVN-Dimethylacetamide (DMAc, 98%) for preparing the membrane casting solution was obtained from Sigma-Aldrich.
- 2-(2-bromoisobutyryloxy) ethyl acrylate (BIEA) was synthesized through the reaction of BIBB and 2-Hydroxyethyl acrylate using a method published previously.
- 34 Phosphate buffer saline (PBS) was purchased from Sigma.
- the prepared PVDF UF membrane of about 40 cm 2 in surface area was pretreated with a continuous stream of O 3 /O 2 mixture.
- the O 3 /O 2 mixture was bubbled through 80 mL isopropanol (IPA) solution with a flow rate of 6 L/min for 30 min and ozone concentration of about 46 g/L at 25° C. which was generated from a custom-built ozone generator (Model OG-10PWA, Ray-E Creative Co., Ltd Taiwan).
- IPA isopropanol
- the ozone-treated PVDF membrane was placed into a 30 mL IPA with 10 wt % BIEA monomer.
- the reactor flask with solution was saturated with purified argon for 5 min and then placed in an oil bath at 80° C. under constant stirring.
- the PBIEA grafted PVDF membrane (PVDF-g-PBIEA) was transferred into purified IPA. Unreacted monomers and homopolymers were extracted with double distilled water and acetone in an ultrasonic washer and the residue solvent was removed in a vacuum oven under reduced pressure.
- the surface copolymerization of SBMA macromonomer on the PVDF-g-PBIEA membrane was prepared via surface-initiated ATRP. A schematic illustration is also shown in FIG. 1 .
- the PVDF-g-PBIEA membrane of about 40 cm 2 in surface area was placed into a 30 mL methanol solution with SBMA macromonomer content adjusted from 0.56 to 5.6 g to achieve the desired grafting density of polySBMA.
- a purified argon stream was introduced to degas the solution in a single-necked round-bottom flask for about 20 min. 316 mg 2,2′-bipyridine and 100 mg CuBr were added sequentially to the solution.
- the chemical composition of surface-modified PVDF membranes with PBIEA and polySBMA was characterized using FT-IR spectrophotometer (Perkin-Elmer Spectrum One) and using Zinc Selenide (ZnSe) as an internal reflection element. Each spectrum was captured by averaged 32 scans at a resolution of 4 cm ⁇ 1 .
- the surface composition of the membranes was also characterized by X-Ray photoelectron spectroscopy (XPS). XPS analysis was performed using a PHI Quantera SXM/Auger spectrometer with a monochromated Al KR X-ray source (1486.6 eV photons).
- the energy of emitted electrons is measured with a hemispherical energy analyzer at pass energies ranging from 50 to 150 eV. All data were collected at the photoelectron takeoff angles of 45° with respect to the sample surface.
- the binding energy (BE) scale is referenced by setting the peak maximum in the C 1s spectrum to 284.6 eV. High-resolution C 1s spectrum was fitted using a Shirley background subtraction and a series of Gaussian peaks. Data analysis software was from Service Physics, Inc.
- the grafting density of polySBMA on the PVDF membrane was determined by the extent of weight increase compared with the virgin PVDF membrane and normalized to the outer surface area of the membranes. Prior to the weight measurements, the membranes were dried overnight in a vacuum oven at 50° C.
- Weight measurements were performed using three independent membranes for each modified membrane, and the average value was reported. Water contact angles were measured with an angle-meter (Automatic Contact Angle Meter, Model CA-VP, Kyowa Interface Science Co., Ltd Japan) at 25° C. The DI water was dropped on the sample surface at ten different sites. The average of the measured values from three independent membranes for each modified membrane was taken as its water contact angle.
- the surface morphology of the surface-modified PVDF UF membranes was observed under JEOL JSM-5410 scanning electron microscopy (SEM) operating at an accelerating voltage of 7 keV. The membranes were mounted on the sample stages by means of double-sided adhesive tape and were sputter-coated with gold prior to SEM.
- the adsorption of BSA and ⁇ -globulin (99%, purchased from Sigma-Aldrich) onto the prepared PVDF UF membranes was evaluated using the method of Bradford according to the standard protocol of the Bio-Rad protein assay.
- the membrane with 20 cm 2 of surface area was rinsed with 20 mL of ethanol for 30 min and transferred into a clean test tube, followed by the addition of 20 mL of PBS solution for 30 min.
- the membrane was soaked in 5 mL of 1 mg/mL BSA and ⁇ -globulin in 0.1 M PBS solution (PH 7.4) for 24 h at 37° C. respectively.
- the membrane was then followed by the addition of dye reagent containing Coomassie Brilliant Blue G-250 and was incubated for 5 min.
- the absorbance at 595 nm was determined by a UV-VIS spectrophotometer.
- a dead-end cell filtration system connected with a nitrogen gas cylinder and solution reservoir was designed to characterize the filtration performance of the prepared membranes.
- the system consisted of a filtration cell (HP4750 stirred cells, Sterlitech Corp.) with a volume capacity of 300 mL and an inner diameter of 49 mm.
- the virgin or prepared membranes were incubated and pressurized with double distilled water for 30 min at 1.5 atm. All the ultrafiltration experiments were operated at a pressure of 1.0 atm, a temperature of 25° C. and a stirring speed of 300 rpm.
- the i th cycle permeation flux (J wi or J Pi ) was checked from time to time until steady and calculated by the following equation:
- V wi , V Pi , A, and ⁇ t denote the pure water and protein solution permeate volume in the i th cycle (in L), membrane area (in m 2 ) and permeation time (in h).
- the filtration cell was emptied and refilled with 1 mg/mL protein solution, and the flux was checked from time to time until a steady flux was obtained.
- the protein rejection ratio (R) was then calculated by the following equation:
- R ⁇ ( % ) ( 1 - C P C f ) ⁇ 100 ⁇ % ( 2 )
- parameters of C p and C f are protein concentration of permeate and feed solution respectively.
- the protein concentration was measured by a UV-VIS Spectroscopy (JASCO V-550, Japan).
- the used membranes were then cleaned by flushing deionized water. To complete a cycle, pure water flux was again measured.
- the degree of water flux recovery during the i th cycle, FR w,i can be calculated by the following equation:
- FR w , i ⁇ ( % ) ( J w , i J w , i - 1 ) ⁇ 100 ⁇ % ( 3 )
- R t , i ⁇ ( % ) ( J w , i - J P , i J w , i ) ⁇ 100 ⁇ % . ( 4 )
- the flux loss was caused by both reversible and irreversible protein fouling in the i th cycle (R r,i and R ir,i ). Each of them was defined by
- R r , i ⁇ ( % ) ( J w , i - J P , i J w , i - 1 ) ⁇ 100 ⁇ % ( 5 )
- the PVDF ultra-filtration membrane was prepared by the wet inversion process. Polyethylene glycol was added in the PVDF casting solution as the pore former. Brush like polySBMA polymer was grafted to resist protein fouling during ultra-filtration. In order to graft the highly polar SBMA onto the hydrophobic surface of PVDF membrane, ozone was used to activate the PVDF and brush like structure was created by surface-initiated ATRP of SBMA.
- the process for surface modification could be divided into three stages.
- the first stage was to produce peroxides on the PVDF membrane via ozone treatment.
- the amount of produced peroxide was controlled by the O 3 concentration and the treating time.
- the peroxide content on PVDF was determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) depletion assay. It was found that thirty minutes ozone treatment generated a peroxide content of 2.35 nmol/cm 2 .
- DPPH 2,2-diphenyl-1-picrylhydrazyl
- the second stage was to graft the initiator, BIEA, onto the surface of PVDF membrane via thermal-induced polymerization.
- the decomposition of peroxides on ozone-treated PVDF membranes was executed by raising the temperature.
- a BIEA grafting time of 24 hours at 80° C. was used in this study.
- the grafted BIEA is identified by XPS measurement.
- FIG. 2( b )-( c ) shows the XPS C 1s and Br 3d core-level spectra of the PVDF-g-PBIEA membrane.
- the C 1s core-level spectra possessed five components via curve fitting.
- the third stage in the preparation process was to copolymerize the macro-monomer of SBMA to the PVDF-g-PBIEA membranes via ATRP.
- the grafted amount of SBMA was controlled by the ratio of SBMA to BIEA in the reaction solution and also by the reaction time of ATRP.
- Atom transfer radical polymerization was activated by the bromide in BIEA under the catalysis of Cu(I)/bpy.
- FT-IR measurement was used to characterize the chemical composition of the polySBMA modified PVDF membrane and its typical spectrum was shown in FIG. 3 .
- the presence of the grafted polymer could be ascertained from the ester carbonyl groups and the sulfonate groups observed from the bands of O—C ⁇ O stretch at 1727 cm ⁇ 1 and —SO 3 stretch at 1033 cm ⁇ 1 , respectively.
- the intensity of the O—C ⁇ O adsorption at 1727 cm ⁇ 1 and —SO 3 at 1033 cm ⁇ 1 increased obviously as the starting SBMA concentration was increased from 0.019 to 0.187 g/mL.
- the grafting density was measured by the weight increase of modified membranes.
- FIG. 4 showed the dependence of surface grafting density and contact angle on the monomer concentration in the reaction solution.
- the grafting density of the copolymerized polySBMA on the PVDF membrane increased, and the water contact angle decreased.
- the grafting density seemed to increase monotonically with the increase of SMBA concentration and finally reached the highest value of 0.4 mg/cm 2 .
- the water contact angle decreased smoothly with the increase of SBMA and reached a lowest value of 52°. Both curves of the PVDF-g-PBIEA-g-PSBMA membrane approached to their optimum values while the SBMA concentration was higher than 0.187 g/mL.
- FIG. 5( a ) is a SEM image of the virgin PVDF membrane.
- the finger-like macrovoids with a top skin layer was observed from the cross-section morphology in the inset in FIG. 5( a ).
- FIG. 5( b ) After the ozone treatment, larger pores were observed on the surface, shown in FIG. 5( b ), which was probably caused by ozone etching.
- FIG. 5( c )-( d ) showed the SEM images of the PVDF membrane grafted with PSBMA of different grafting densities.
- the measurement of protein adsorption has already become a good indication of the filtration performance of membrane for protein processing.
- Two major plasma proteins, albumin and ⁇ -globulin were selected to test in this study.
- the protein adsorption was evaluated by immersing the membranes in 1 mg/mL BSA or ⁇ -globulin solution and the amount of adsorbed protein was estimated by measuring the protein concentration in the supernatant.
- FIG. 6 showed the effect of grafting density on the amount of BSA or ⁇ -globulin adsorption. All of the adsorption data were normalized to the apparent surface area of the measured membranes. It was found that both the adsorptions of BSA and ⁇ -globulin decreased linearly with the grafting density and the slopes were almost identical.
- FIG. 7 shows the flux of PVDF membranes grafted with polySBMA of different grafting densities. It was found that the J W0 of the membrane grafted with 0.25 mg/cm 2 polySBMA was actually higher than that of the virgin PVDF membrane. The higher flux was probably due to the etching caused by ozone treatment. It was also found that the J W0 values decreased as the amount of grafted polySBMA was further increased. Each filtration cycle could be divided into three phases. The first phase was the filtration of protein solution.
- the second phase was simple membrane cleaning by pure water flushing.
- the third phase was the passing of pure water.
- the permeation flux of BSA solution decreased rapidly at the initial stage because of protein fouling.
- a steady flux (J Pi ) was obtained when the protein adsorption became saturated.
- the pure water flux (J wi ) was measured after membrane cleaning.
- the values of J wi and J Pi could then be used to calculate water flux recovery, total fouling, reversible and irreversible fouling ratios.
- FIG. 8 shows these ratios in the first cycle using BSA as the tested protein.
- the total protein fouling (R t,i ) was calculated by equation (4) which represented the percentage flux loss because of protein adsorption and retention.
- the total protein fouling was actually built up by persistent protein adsorption and temporal protein blockage.
- the flux loss because of temporal protein blockage was reversible and could be recovered by membrane cleaning. Therefore, the reversible fouling ratio, R r,i , was defined by the percentage of pure water flux recovered in the third phase from its loss in the first phase.
- the water flux recovery ratio, FR w,i defined as the ratio of pure water flux in the i th cycle to that in the previous cycle, was exactly equal to 1 ⁇ R ir,i .
- the antifouling capability of a membrane was usually monitored by the value of R ir,i or FR w,i .
- the higher value of FR w,i indicated the lower persistent protein adsorption to the membrane operated during the i th cycle.
- the first cycle analysis was shown in FIG. 8 .
- the value of FR w,1 was only 28.3% for the untreated PVDF membrane, but it increased as the grafting density increased. The value had a sudden increase at the grafting density of 0.35 mg/cm 2 and further increased to 88.8% at the grafting density of 0.4 mg/cm 2 .
- the first cycle analysis of BSA filtration indicated that the PVDF membranes grafted with polySBMA effectively reduced the membrane fouling.
- the surface of PVDF could be fully covered only when the grafting density was increased to above 0.35 mg/cm 2 .
- the R t,1 value represented the overall effect of fouling on flux loss. The loss may be contributed by both the loosely attached and the firmly adsorbed proteins. It was observed that the R t,1 decreased with the increase in polySBMA grafting density and there was a sudden drop of R t,1 at the grafting density of 0.35 mg/cm 2 . The R r,1 value was contributed by the loosely attached proteins.
- FIG. 10 shows the results of ⁇ -globulin filtration through the PVDF-g-polySBMA membranes. Unlike the BSA filtration, the results of ⁇ -globulin filtration showed that both the virgin and modified membranes were continuously fouled by ⁇ -globulin. The permeation fluxes were decreased and the rejection ratio increased after each cyclic operation. In spite of its continuous fouling, the performance of the PVDF-g-polySBMA membrane was still much better than that of the virgin PVDF.
- FIG. 11 shows the water flux recovery ratio of these two membranes. For the virgin PVDF membrane, the water flux recovery ratio was 18.7% in the first cycle. The value was increased to 76% in the second cycle and reached 79% in the third cycle.
- the water flux recovery ratio was 74.7% in the first cycle.
- the value was increased to 91.4% in the second cycle and reached 95.3% in the third cycle.
- It reflected an irreversible fouling ratio of 25.3% in the first cycle, 8.6% and 4.7% in the second and third cycle.
- the modified membrane was still continuously fouled by ⁇ -globulin. But the degree of irreversible fouling was significantly reduced after the first cycle. There may be two possible reasons for continuous fouling.
- PVDF UF membranes were successfully grafted with zwitterionic PSBMA polymer through ozone activated surface-treatment and surface-initiated atom transfer radical polymerization.
- the membrane grafted with 0.4 mg/cm 2 PSMA hardly adsorbs BSA but adsorbs a small amount of ⁇ -globulin of 10 ⁇ g/cm 2 .
- the cyclic filtration test of the membrane using BSA solution showed perfect non-fouling characteristics.
- the water flux recovery was 88.9% in the first cycle and reached 100% in the second cycle.
- the similar test on ⁇ -globulin showed some but low protein fouling.
- the water flux recovery was 73.7% in the first cycle and reached 95.5% in the third cycle.
- the low-fouling characteristics suggested that the PSMBA grafting by the suggested method, ozone surface activation followed by surface-initiated atom transfer radical polymerization, could actually penetrate into the pores.
- the cyclic filtration test of BSA alone might not be sufficient to demonstrate the protein fouling to the membranes, in stead, the test on ⁇ -globulin gave a closer look.
- the low biofouling filtration membrane provided in this invention can be used in membrane bioreactor.
- Membrane bioreactor usually is a combination of conventional wastewater treatment and a membrane filtration process, has acquired considerable attention in the development of alternative water resources as a result of the following advantages: complete solids removal, significant physical disinfection capability, superior organic and nutrient removals and small footprint.
- membrane fouling in the MBR which increases the operational cost, limits their usage.
- Membrane fouling is a ubiquitous phenomenon in MBRs, which is mainly caused by microbial substances. In a MBR process containing a variety of bacteria, the bacterial adhesion to the membrane surface, prior to cake formation, causes an increased filtration resistance.
- the above-mentioned fluorine-based filtration membranes with grafted zwitterionic group can remarkably reduce the bacterial adhesion, so as to obtain higher filtrate.
- the low biofouling filtration membrane provided in this invention can also be used in the purification, concentration or separation of stem cells.
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Abstract
Description
where the parameters: Vwi, VPi, A, and Δt denote the pure water and protein solution permeate volume in the ith cycle (in L), membrane area (in m2) and permeation time (in h). For each cyclic operation, the filtration cell was emptied and refilled with 1 mg/mL protein solution, and the flux was checked from time to time until a steady flux was obtained. The protein rejection ratio (R) was then calculated by the following equation:
where parameters of Cp and Cf are protein concentration of permeate and feed solution respectively. The protein concentration was measured by a UV-VIS Spectroscopy (JASCO V-550, Japan). The used membranes were then cleaned by flushing deionized water. To complete a cycle, pure water flux was again measured. The degree of water flux recovery during the ith cycle, FRw,i, can be calculated by the following equation:
The flux loss was caused by both reversible and irreversible protein fouling in the ith cycle (Rr,i and Rir,i). Each of them was defined by
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